Application of a colorimetric CTLL-2 cell proliferation assay for the evaluation of IL-15 antagonists

Dysregulated expression of Interleukin (IL)-15 has been associated to several autoimmune and inflammatory diseases. In this sense, some agents that inhibit IL-15 activity have been developed as possible drugs to treat these pathologies. We have been working in two strategies to inhibit IL-15 activity, a peptide (named P8) that binds to the alpha subunit of the IL-15 receptor (IL-15R) and a vaccine based on active immunization with human IL-15 (hIL-15). To measure the biological activity of the IL-15 antagonists we used the proliferation assay in CTLL-2, a cell line that depends on IL-2/ IL-15 to proliferate. In the current work we defined the conditions of the assay to determine the half maximal inhibitory concentration (IC50) of the P8 peptide and the neutralizing titers of the sera from monkeys immunized with the anti-IL-15 vaccine. The specificity of the assay for IL-15 was documented using anti-IL-15 and anti-IL-2 specific antibodies. We also examined the specificity of the antagonists of IL-15 in presence of IL-2; neither the peptide nor the sera inhibited the cell proliferation induced by human IL-2. Finally, we evaluated the effect of commercial antibodies anti-hIL-15 and sera from mice immunized with hIL-15 on murine IL-15 (mIL-15). In this case, neither the antibodies nor the sera inhibited mIL-15; therefore, murine models are not useful to evaluate the effectiveness of anti-hIL-15 vaccine. The application of this assay allowed us to evaluate different strategies to inhibit the activity of IL-15 for its future development as drugs